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Abstract:

In an internal combustion engine that causes a predetermined gas flow in
a combustion chamber, discharge plasma generated by a discharge device is
caused to effectively absorb energy of an electromagnetic wave emitted
from an electromagnetic wave emission device. At a time when a discharge
operation and an emission operation are simultaneously performed so as to
ignite a fuel air mixture, an emitting position of the electromagnetic
wave on an antenna during the emission operation is located downstream of
the discharge gap in a direction of the gas flow at the discharge gap so
as to face toward the discharge plasma that has been drifted due to the
gas flow.

Claims:

1. An internal combustion engine comprising: an internal combustion
engine main body configured to produce a predetermined gas flow in a
combustion chamber; a discharge device that generates discharge plasma at
a discharge gap located in the combustion chamber; and an electromagnetic
wave emission device including an electromagnetic wave oscillator that
oscillates an electromagnetic wave, and an antenna for emitting the
electromagnetic wave supplied from the electromagnetic wave oscillator to
the combustion chamber, wherein the internal combustion engine that
simultaneously performs a discharge operation of causing the discharge
device to generate the discharge plasma and an emission operation of
emitting the electromagnetic wave from the antenna by driving the
electromagnetic wave oscillator, thereby supplying the discharge plasma
with energy of the electromagnetic wave so as to ignite fuel air mixture
in the combustion chamber, and the electromagnetic wave emission device
is configured such that an emitting position of the electromagnetic wave
on the antenna during the emission operation is located downstream of the
discharge gap in a direction of the gas flow at the discharge gap so as
to face toward the discharge plasma that has been drifted due to the gas
flow.

2. The internal combustion engine according to claim 1, wherein an
antenna having an emitting position of the electromagnetic wave during
the emission operation, the emitting position facing toward the discharge
plasma that has been drifted due to the gas flow, is assumed as a first
antenna, the electromagnetic wave emission device includes, in addition
to the first antenna, a second antenna for emitting an electromagnetic
wave, and an emitting position on the second antenna during the emission
operation is located downstream of the discharge gap further away from
the emitting position of the electromagnetic wave on the first antenna in
relation to the direction of the gas flow at the discharge gap so that
the electromagnetic wave emitted at a time of the discharge operation
from the second antenna creates a strong electric field region, which has
an electric field relatively strong in intensity in the combustion
chamber, in a region adjacent to the discharge plasma that has been
drifted due to the gas flow.

3. The internal combustion engine according to claim 2, wherein the
electromagnetic wave emission device includes a third antenna located
further away from the discharge device than the first antenna and the
second antenna, and generates electromagnetic wave plasma by emitting the
electromagnetic wave from the third antenna to a region not yet reached
by a flame surface after the fuel air mixture has been ignited.

4. The internal combustion engine according to claim 3, wherein the third
antenna is located so as to emit the electromagnetic wave to a region
where occurrence frequency of knocking is relatively high in the
combustion chamber

5. The internal combustion engine according to claim 1, wherein the
electromagnetic wave emission device emits the electromagnetic wave from
the antenna to a region which the flame surface has passed through after
the fuel air mixture has been ignited.

6. The internal combustion engine according to claim 1, wherein the
emitting position of the electromagnetic wave on the antenna is covered
by an insulator or dielectric material.

7. The internal combustion engine according to claim 1, wherein a pair of
floating electrodes are provided facing toward each other on opposite
sides with respect to an imaginary line passing through the discharge gap
and the emitting position of the electromagnetic wave on the antenna
during the emission operation.

8. An internal combustion engine comprising: an internal combustion
engine main body configured to produce a predetermined gas flow in a
combustion chamber; a discharge device that generates discharge plasma at
a discharge gap located in the combustion chamber; and an electromagnetic
wave emission device including an electromagnetic wave oscillator that
oscillates an electromagnetic wave, and an antenna for emitting the
electromagnetic wave supplied from the electromagnetic wave oscillator to
the combustion chamber, wherein the internal combustion engine
simultaneously performs a discharge operation of causing the discharge
device to generate the discharge plasma and an emission operation of
emitting the electromagnetic wave from the antenna by driving the
electromagnetic wave oscillator, thereby supplying the discharge plasma
with energy of the electromagnetic wave so as to ignite a fuel air
mixture in the combustion chamber, and the electromagnetic wave emission
device is configured such that an emitting position of the
electromagnetic wave on the antenna during the emission operation is
located downstream of the discharge gap in a direction of the gas flow at
the discharge gap so that the electromagnetic wave emitted from the
antenna at a time of the discharge operation creates a strong electric
field region, which has an electric field relatively strong in intensity
in the combustion chamber, in a region adjacent to the discharge plasma
that has been drifted due to the gas flow.

Description:

TECHNICAL FIELD

[0001] The present invention relates to an internal combustion engine
provided with an electromagnetic wave emission device that emits an
electromagnetic wave to a combustion chamber.

BACKGROUND ART

[0002] Conventionally, there is known an internal combustion engine
provided with an electromagnetic wave emission device that emits an
electromagnetic wave to a combustion chamber. For example, Japanese
Unexamined Patent Application, Publication No. 2009-36198 discloses an
internal combustion engine of this kind. Japanese Unexamined Patent
Application, Publication No. 2009-36198 discloses an internal combustion
engine provided with an ignition or plasma generation device. The
ignition or plasma generation device mixes a high voltage pulse with a
microwave pulse and supplies them to an ignition plug, thereby igniting
fuel air mixture in a combustion chamber.

[0003] Furthermore, conventionally, there is known an internal combustion
engine configured to produce a predetermined gas flow such as a tumble
flow in a combustion chamber. This kind of internal combustion engine
agitates a fuel air mixture by the gas flow so as to ensure a uniform
distribution of the fuel.

THE DISCLOSURE OF THE INVENTION

Problems to be Solved by the Invention

[0004] In the internal combustion engine that produces the predetermined
gas flow (such as the tumble flow), discharge plasma generated by a
discharge device is drifted due to the gas flow. Therefore, even if the
electromagnetic wave emission device is applied to this kind of internal
combustion engine, and a discharge gap is irradiated with the
electromagnetic wave, since a major part of the discharge plasma does not
remain in the discharge gap, it is impossible to cause the discharge
plasma to effectively absorb energy of the electromagnetic wave. For
example, in a case in which the electromagnetic wave is emitted to the
combustion chamber so as to expand a lean limit, it is impossible to
fully expand the lean limit.

[0005] The present invention has been made in view of the above described
circumstances, and it is an object of the present invention, in an
internal combustion engine that produces a predetermined gas flow in a
combustion chamber, to cause discharge plasma generated by a discharge
device to effectively absorb energy of an electromagnetic wave emitted
from an electromagnetic wave emission device.

Means for Solving the Problems

[0006] In accordance with a first aspect of the present invention, there
is provided an internal combustion engine including: an internal
combustion engine main body configured to produce a predetermined gas
flow in a combustion chamber; a discharge device that generates discharge
plasma at a discharge gap located in the combustion chamber; and an
electromagnetic wave emission device including an electromagnetic wave
oscillator that oscillates an electromagnetic wave, and an antenna for
emitting the electromagnetic wave supplied from the electromagnetic wave
oscillator to the combustion chamber. The internal combustion engine that
simultaneously performs a discharge operation of causing the discharge
device to generate the discharge plasma and an emission operation of
emitting the electromagnetic wave from the antenna by driving the
electromagnetic wave oscillator, thereby supplying the discharge plasma
with energy of the electromagnetic wave so as to ignite a fuel air
mixture in the combustion chamber. The electromagnetic wave emission
device is configured such that an emitting position of the
electromagnetic wave on the antenna during the emission operation is
located downstream of the discharge gap in a direction of the gas flow at
the discharge gap so as to face toward the discharge plasma that has been
drifted due to the gas flow.

[0007] According to the first aspect of the present invention, the
discharge operation and the emission operation are simultaneously
performed. The discharge device generates the discharge plasma. On the
other hand, the electromagnetic wave emission device emits the
electromagnetic wave from the antenna to the combustion chamber. During
the emission operation, the emitting position (radiating position) of the
electromagnetic wave on the antenna is located downstream of the
discharge gap in the direction of the gas flow at the discharge gap so as
to face toward the discharge plasma that has been drifted due to the gas
flow. The discharge plasma that has been drifted due to the gas flow is
present at a location which the electromagnetic wave is emitted to.
Accordingly, the discharge plasma effectively absorbs the energy of the
electromagnetic wave emitted from the antenna.

[0008] In accordance with a second aspect of the present invention, in
addition to the first aspect of the present invention, an antenna having
an emitting position of the electromagnetic wave during the emission
operation, the emitting position facing toward the discharge plasma that
has been drifted due to the gas flow, is assumed as a first antenna, the
electromagnetic wave emission device includes, in addition to the first
antenna, a second antenna for emitting an electromagnetic wave, and an
emitting position on the second antenna during the emission operation is
located downstream of the discharge gap further away from the emitting
position of the electromagnetic wave on the first antenna in relation to
the direction of the gas flow at the discharge gap so that the
electromagnetic wave emitted at a time of the discharge operation from
the second antenna creates a strong electric field region, which has an
electric field relatively strong in intensity in the combustion chamber,
in a region adjacent to the discharge plasma that has been drifted due to
the gas flow.

[0009] According to the second aspect of the present invention, at the
time of the discharge operation, the second antenna emits the
electromagnetic wave so as to create the strong electric field region in
the region adjacent to the discharge plasma that has absorbed the energy
of the electromagnetic wave emitted from the first antenna. The discharge
plasma reacts with the electric field of the strong electric field
region. The discharge plasma is supplied with the energy of the
electromagnetic wave from the first and second antennae.

[0010] In accordance with a third aspect of the present invention, in
addition to the second aspect of the present invention, the
electromagnetic wave emission device includes a third antenna located
further away from the discharge device than the first antenna and the
second antenna, and generates electromagnetic wave plasma by emitting the
electromagnetic wave from the third antenna to a region not yet reached
by a flame surface after the fuel air mixture has been ignited.

[0011] According to the third aspect of the present invention, during a
flame propagation after the fuel air mixture has been ignited, the
electromagnetic wave plasma is generated in the region not yet reached by
the flame surface. The electromagnetic wave plasma generates active
species. The flame surface passes through a region in which the active
species are generated.

[0012] In accordance with a fourth aspect of the present invention, in
addition to the third aspect of the present invention, the third antenna
is located so as to emit the electromagnetic wave to a region where
occurrence frequency of knocking is relatively high in the combustion
chamber.

[0013] According to the fourth aspect of the present invention, in a
region where occurrence frequency of knocking is relatively high in the
combustion chamber, active species are generated before the flame surface
reaches the region. When knocking is caused, the speed of the flame
surface is decreased before the flame surface reaches a region where
knocking is likely to be caused, thereby resulting in the knocking.
According to the fourth aspect of the present invention, the active
species are generated in the region where knocking is likely to be
caused. Therefore, the speed of the flame surface is prevented from
decreasing.

[0014] In accordance with a fifth aspect of the present invention, in
addition to the first aspect of the present invention, the
electromagnetic wave emission device emits the electromagnetic wave from
the antenna to a region which the flame surface has passed through after
the fuel air mixture has been ignited.

[0015] According to the fifth aspect of the present invention, after the
fuel air mixture has been ignited, the antenna emits the electromagnetic
wave to the region which the flame surface has passed through. In the
region after the flame has passed through, temperature rises due to the
electromagnetic wave, and oxidation reaction of the fuel air mixture is
promoted. Furthermore, pressure rises behind the flame surface, and flame
propagation is promoted.

[0016] In accordance with a sixth aspect of the present invention, in
addition to the first aspect of the present invention, the emitting
position of the electromagnetic wave on the antenna is covered by an
insulator or dielectric material.

[0017] In accordance with a seventh aspect of the present invention, in
addition to the first aspect of the present invention, a pair of floating
electrodes are provided facing toward each other on opposite sides with
respect to an imaginary line passing through the discharge gap and the
emitting position of the electromagnetic wave on the antenna during the
emission operation.

[0018] According to the seventh aspect of the present invention, it is
difficult for the discharge plasma generated at the discharge gap to
drift toward either side of the floating electrodes.

[0019] In accordance with an eighth aspect of the present invention, there
is provided an internal combustion engine including: an internal
combustion engine main body configured to produce a predetermined gas
flow in a combustion chamber; a discharge device that generates discharge
plasma at a discharge gap located in the combustion chamber; and an
electromagnetic wave emission device including an electromagnetic wave
oscillator that oscillates an electromagnetic wave, and an antenna for
emitting the electromagnetic wave supplied from the electromagnetic wave
oscillator to the combustion chamber. The internal combustion engine
simultaneously performs a discharge operation of causing the discharge
device to generate the discharge plasma and an emission operation of
emitting the electromagnetic wave from the antenna by driving the
electromagnetic wave oscillator, thereby supplying the discharge plasma
with energy of the electromagnetic wave so as to ignite a fuel air
mixture in the combustion chamber. The electromagnetic wave emission
device is configured such that an emitting position of the
electromagnetic wave on the antenna during the emission operation is
located downstream of the discharge gap in a direction of the gas flow at
the discharge gap so that the electromagnetic wave emitted from the
antenna at a time of the discharge operation creates a strong electric
field region, which has an electric field relatively strong in intensity
in the combustion chamber, in a region adjacent to the discharge plasma
that has been drifted due to the gas flow.

[0020] According to the eighth aspect of the present invention, at the
time of the discharge operation, the antenna emits the electromagnetic
wave so as to create the strong electric field region at the region
adjacent to the discharge plasma. The discharge plasma reacts with the
electric field of the strong electric field region. The discharge plasma
is supplied with the energy of the electromagnetic wave from the antenna.

EFFECT OF THE INVENTION

[0021] According to the present invention, since the discharge plasma,
which has been drifted due to the gas flow, is present at a location
which the electromagnetic wave is emitted to, the discharge plasma can
effectively absorb the energy of the electromagnetic wave. Accordingly,
in comparison with a conventional internal combustion engine that is not
configured in view of the fact that the discharge plasma may be drifted,
it is possible to increase the energy of the electromagnetic wave
absorbed by the discharge plasma, thereby making it possible to reduce
the intensity of the reflection electromagnetic wave. Therefore, it is
possible to attain ignition stability even if lean fuel air mixtures are
in use.

[0022] Furthermore, according to the second aspect of the present
invention, since the discharge plasma is supplied with the energy of the
electromagnetic wave from the first and second antennae, it is possible
to supply a larger amount of energy to the discharge plasma.

[0023] Furthermore, according to the third aspect of the present
invention, it is configured such that the active species are generated in
a region not yet reached by the flame surface to ensure that the flame
surface passes through the region in which the active species are
generated. Accordingly, oxidation reaction on the flame surface is
promoted by the active species, and it is possible to improve the
propagation speed of the flame surface. Therefore, it is possible to
reduce an amount of unburned fuel in a case in which a lean fuel air
mixture is burned.

[0024] Furthermore, according to the fourth aspect of the present
invention, the speed of the flame surface is prevented from decreasing
before the flame surface reaches a region in which knocking is likely to
be caused. Therefore, it is possible to cause the flame to reach a region
in which knocking is likely to be caused before knocking is actually
caused, thereby making it possible to suppress the occurrence of
knocking.

[0025] Furthermore, according to the fifth aspect of the present
invention, it is configured such that the electromagnetic wave is emitted
to a region which the flame has passed through so as to promote the
oxidation reaction of the fuel air mixture, thereby promoting the flame
propagation from behind the flame surface. Therefore, it is possible to
reduce an amount of unburned fuel in a case in which a lean fuel air
mixture is burned.

[0026] Furthermore, according to the seventh aspect of the present
invention, it is difficult for the discharge plasma generated at the
discharge gap to drift toward either side of the floating electrodes,
thereby maintaining a drift direction of the discharge plasma constant to
some extent. As a result thereof, it is possible to increase a period of
time, in which the emitting position of the electromagnetic wave on the
antenna can face toward the discharge plasma. Accordingly, it is possible
to increase the amount of energy of the electromagnetic wave absorbed by
the discharge plasma.

[0027] Furthermore, according to the eighth aspect of the present
invention, the discharge plasma is supplied with the energy of the
electromagnetic wave from the antenna. Therefore, it is possible to
attain ignition stability even if lean fuel air mixtures are in use.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] FIG. 1 is a schematic configuration diagram of an internal
combustion engine according to an embodiment;

[0029] FIG. 2 is a block diagram of a discharge device and an
electromagnetic wave emission device according to the embodiment;

[0030] FIG. 3 is a schematic configuration diagram of a relevant part of
the internal combustion engine according to the embodiment, FIG. 3a
showing a first antenna emitting a microwave, FIG. 3b showing a second
antenna emitting a microwave, and FIG. 3c showing a third antenna
emitting a microwave;

[0031]FIG. 4 is a front view of a ceiling surface of a combustion chamber
of the internal combustion engine according to the embodiment;

[0032] FIG. 5 is a time chart illustrating microwave emission period from
each antenna and the like according to the embodiment;

[0033] FIG. 6 is a front view of a ceiling surface of a combustion chamber
of an internal combustion engine according to a first modified example of
the embodiment;

[0034] FIG. 7 is a front view of a ceiling surface of a combustion chamber
of an internal combustion engine according to a second modified example
of the embodiment;

[0035] FIG. 8 is a front view of a ceiling surface of a combustion chamber
of an internal combustion engine according to a fourth modified example
of the embodiment;

[0036] FIG. 9 is a schematic configuration diagram of a relevant part of
the internal combustion engine according to a fifth modified example of
the embodiment, FIG. 9a showing a case of a first antenna not emitting a
microwave, and FIG. 9b showing a case of the first antenna emitting a
microwave;

[0037] FIG. 10 is a schematic configuration diagram of a relevant part of
the internal combustion engine according to a sixth modified example of
the embodiment

[0038] FIG. 11 is a front view of a ceiling surface of a combustion
chamber of an internal combustion engine according to a seventh modified
example of the embodiment; and

[0039] FIG. 12 is a front view of a ceiling surface of a combustion
chamber of an internal combustion engine according to an eighth modified
example of the embodiment.

BEST MODE FOR CARRYING OUT THE INVENTION

[0040] In the following, a detailed description will be given of the
embodiment of the present invention with reference to drawings. It should
be noted that the following embodiment is a mere example that is
essentially preferable, and is not intended to limit the scope of the
present invention, applied field thereof, or application thereof.

Embodiment

[0041] The present embodiment is directed to an internal combustion engine
10 including an electromagnetic wave emission device 13 that emits an
electromagnetic wave to a combustion chamber 20. The internal combustion
engine 10 is a reciprocating type engine in which a piston 23
reciprocates. The internal combustion engine 10 is provided with an
internal combustion engine main body 11, a discharge device 12, and the
electromagnetic wave emission device 13. The internal combustion engine
10 is controlled by an electronic control device 30 (ECU).

<Internal Combustion Engine Main Body>

[0042] As shown in FIG. 1, the internal combustion engine main body 11 is
provided with a cylinder block 21, a cylinder head 22, and the pistons
23. The cylinder block 21 is formed with a plurality of cylinders 24 each
having a circular cross section. Inside of each cylinder 24, the piston
23 is reciprocatably mounted. The piston 23 is connected to a crankshaft
(not shown) via a connecting rod (not shown). The crankshaft is rotatably
supported by the cylinder block 21. While the piston 23 reciprocates in
each cylinder 24 in an axial direction of the cylinder 24, the connecting
rod converts the reciprocation movement of the piston 23 into rotation
movement of the crankshaft.

[0043] The cylinder head 22 is placed on the cylinder block 21, and a
gasket 18 intervenes between the cylinder block 21 and the cylinder head
22. The cylinder head 22 partitions the combustion chamber 20 along with
the cylinder 24 and the piston 23.

[0044] The cylinder head 22 is provided with one spark plug 15 for each
cylinder 24. The spark plug 15 is attached to the cylinder head 22 so
that a discharge gap between a central electrode 15a and a ground
electrode 15b is located in the combustion chamber 20.

[0045] The cylinder head 22 is formed with an intake port 25 and an
exhaust port 26 for each cylinder 24. The intake port 25 is provided with
an intake valve 27 for opening and closing the intake port 25, and an
injector 29 for injecting fuel. On the other hand, the exhaust port 26 is
provided with an exhaust valve 28 for opening and closing the exhaust
port 26.

[0046] The internal combustion engine 10 is designed such that the intake
port 25 forms a strong tumble flow 35 in the combustion chamber 20. The
tumble flow 35 serves as the predetermined gas flow 35 produced in the
internal combustion engine 10. In the combustion chamber 20, a fuel air
mixture that has flowed in from the intake port 25 flows toward the
direction of the exhaust port 26 along a ceiling surface of the
combustion chamber 20 (a surface of the cylinder head 22 exposed to the
combustion chamber 20), and the flow rotates in a vertical direction
along a wall surface of the cylinder 24 and a top surface of the piston
23. The tumble flow 35 is produced during an intake stroke and a
compression stroke.

<Discharge Device>

[0047] The discharge device 12 is provided in association with each
combustion chamber 20. As shown in FIG. 2, the discharge device 12
includes an ignition coil (a pulse output part) 14 that outputs a high
voltage pulse, and an ignition plug (a discharge generation part) 15 that
causes discharge when applied with the high voltage pulse from the
ignition coil 14.

[0048] The ignition coil 14 is connected to a direct current power supply
(not shown) such as a battery of a vehicle. The ignition coil 14, upon
receiving an ignition signal from the electronic control device 30,
boosts a voltage applied from the direct current power supply, and
outputs the boosted high voltage pulse to the ignition plug 15. The
ignition plug 15, upon application of the high voltage pulse, causes
insulation breakage at the discharge gap so as to cause a spark
discharge. The spark discharge generates discharge plasma 36.

[0049] As described above, the strong tumble flow 35 is formed during the
intake stroke and the compression stroke in the combustion chamber 20, as
shown in FIG. 3. At an ignition timing when the piston 23 locates before
a compression top dead center, a bulk flow of the fuel air mixture at the
discharge gap flows from the side of the intake port 25 to the side of
the exhaust port 26 under influence of the tumble flow 35. As a result of
this, the discharge plasma 36 generated by the spark discharge is drifted
toward the side of the exhaust port 26. The discharge plasma 36 is
extended due to the gas flow 35.

[0050] According to the present embodiment, as shown in FIG. 4, a
connecting part of the ground electrode 15b, which extends in an axial
direction of the ignition plug 15 (a base end side part of the ground
electrode 15b), locates toward a side of a middle region between an
opening part 25a of the intake port 25 and an opening part 26a of the
exhaust port 26. As a result of this, the gas flow 35 at the discharge
gap is hardly influenced by the connecting part. A direction of the gas
flow 35 at the discharge gap is approximately directed toward a midpoint
between the opening parts 26a of the two exhaust ports 26.

[0051] Therefore, the discharge plasma 36 is drifted approximately toward
the midpoint between the opening parts 26a of the two exhaust ports 26.
Actually, the discharge plasma 36 is drifted toward the direction of a
first antenna 41, which will be described later, due to the tumble flow
35.

<Electromagnetic Wave Emission Device>

[0052] As shown in FIG. 2, the electromagnetic wave emission device 13
includes a power supply for electromagnetic wave 31, an electromagnetic
wave oscillator 32, a distributor 33, and a plurality of antennae 41 to
43. According to the present embodiment, three antennae 41 to 43 are
provided for each combustion chamber 20. FIG. 2 shows only the antennae
41 to 43 corresponding to one combustion chamber 20.

[0053] The power supply for electromagnetic wave 31, upon receiving an
electromagnetic wave drive signal from the electronic control device 30,
supplies a pulse current to the electromagnetic wave oscillator 32. The
electromagnetic wave drive signal is a pulse signal. The power supply for
electromagnetic wave 31 outputs the pulse current at a predetermined duty
cycle during a period starting from a rising timing of the
electromagnetic wave drive signal until a falling timing thereof. The
pulse current is continuously outputted during a time period of a pulse
width of the electromagnetic wave drive signal.

[0054] The electromagnetic wave oscillator 32 is, for example, a
magnetron. The electromagnetic wave oscillator 32, upon receiving the
pulse current, outputs a microwave pulse. The electromagnetic wave
oscillator 32 continuously outputs the microwave pulse during the time
period of the pulse width of the electromagnetic wave drive signal. In
place of the magnetron, other types of oscillators such as a
semiconductor oscillator may be employed.

[0055] The distributor 33 switches the antenna to be supplied with the
microwave outputted from the electromagnetic wave oscillator 32, from
among the three antennae 41 to 43. The distributor 33, upon receiving a
distribution signal from the electronic control device 30, supplies the
microwave to the three antennae 41 to 43 in turn.

[0056] As shown in FIG. 3, the three antennae 41 to 43 consist of the
first antenna 41, a second antenna 42, and a third antenna 43, seen from
the side of the ignition plug 15. Each of antennae 41 to 43 may be, for
example, a monopole antenna. A tip end of each of antennae 41 to 43
corresponds to an emitting position (radiating position) of the
microwave.

[0057] The first and second antennae 41 and 42 are embedded in the
cylinder head 22. Emitting ends of the microwave (tip ends) of the first
and second antennae 41 and 42 are slightly protruded from a surface of
the cylinder head 22 (the ceiling surface of the combustion chamber 20).
As shown in FIG. 4, the emitting ends of the first and second antennae 41
and 42 are located in the middle between the opening parts 26a of the two
exhaust ports 26. The emitting ends of the first and second antennae 41
and 42 are arranged along a radial direction of the combustion chamber
20.

[0058] The third antenna 43 is embedded in the gasket 18, and provided
with an emitting end of microwave, which is approximately flush with an
inner periphery of the gasket 18. The third antenna 43 is provided
further away from the discharge device 12 than the first and second
antennae 41 and 42.

[0059] An input end (a base end) of each of antennae 41 to 43 is connected
to the distributor 33. From the emitting end of each of antennae 41 to
43, the microwave supplied from the distributor 33 is emitted to the
combustion chamber 20.

[0060] According to the present embodiment, during an ignition operation,
which will be described later, the emitting end of the first antenna 41
is located downstream of the discharge gap in the direction of the gas
flow 35 at the discharge gap so that the discharge plasma 36 that has
been drifted due to the tumble flow 35 is irradiated with the microwave.
The emitting end of the first antenna 41 is located in the vicinity of
the ignition plug 15 on the ceiling surface of the combustion chamber 20.
The emitting end of the first antenna 41 faces toward a flexure part of
the discharge plasma 36, which is located at a position most distant from
the discharge gap (a part that has been drifted furthest away by the
tumble flow 35). Here, the emitting end of the first antenna 41 faces
toward the flexure part of the discharge plasma 36 throughout an entire
operating range of generating the microwave plasma during the ignition
operation.

[0061] According to the present embodiment, the second and third antennae
42 and 43 are disposed on the same side as the first antenna 41 in
relation to the ignition plug 15. However, the second and third antennae
42 and 43 maybe disposed on a side opposite to the first antenna 41 in
relation to the ignition plug 15.

<Ignition Operation>

[0062] The ignition operation of the fuel air mixture performed by the
discharge device 12 and the electromagnetic wave emission device 13 will
be described hereinafter. During the ignition operation, a discharge
operation of the discharge device 12 for generating the discharge plasma
36 and an emission operation of emitting the microwave from the first
antenna 41 by driving the electromagnetic wave oscillator 32 are
simultaneously performed so that the discharge plasma 36 is supplied with
energy of the microwave, thereby igniting the fuel air mixture in the
combustion chamber 20.

[0063] During the ignition operation, the electronic control device 30
outputs the ignition signal and the electromagnetic wave drive signal.
Then, in the discharge device 12, the ignition coil 14 outputs the high
voltage pulse at a falling timing of the ignition signal, and the spark
discharge is caused at the ignition plug 15. On the other hand, in the
electromagnetic wave emission device 13, the power supply for
electromagnetic wave 31 continuously outputs the pulse current during a
period starting from a rising timing of the electromagnetic wave drive
signal until a falling timing thereof. Subsequently, the electromagnetic
wave oscillator 32, upon receiving the pulse current, continuously
oscillates the microwave pulse and outputs it to the distributor 33. Due
to an operation delay of the magnetron 32, a start and an end of an
oscillation period of the microwave are slightly delayed in relation to a
start and an end of an output period of the pulse current, respectively.

[0064] During the ignition operation, the ignition signal and the
electromagnetic wave drive signal are outputted so that the spark
discharge is caused immediately after the start of the oscillation period
of the microwave, as shown in FIG. 5. During the oscillation period of
the microwave, firstly, the distributor 33 specifies the first antenna 41
as a supply destination of the microwave pulse. The microwave is emitted
from the first antenna 41 to the combustion chamber 20. At a timing when
the spark discharge is caused, a strong electric field region 51, which
has an electric field relatively strong in intensity in the combustion
chamber 20, is formed in the vicinity of the emitting end of the first
antenna 41. As shown in FIG. 3a, the discharge plasma 36 caused by the
spark discharge is drifted due to the strong tumble flow 35 toward the
side of the exhaust ports 26, and the flexure part thereof enters into
the strong electric field region 51. The flexure part of the discharge
plasma 36 is irradiated with the microwave. The discharge plasma 36
absorbs the energy of the microwave and thickens, and consequently forms
a relatively large scale of microwave plasma in the strong electric field
region 51. In the strong electric field region 51, the fuel air mixture
in the combustion chamber 20 is volume ignited due to the microwave
plasma. Subsequently, a flame surface expands outwardly from an ignition
location toward the wall surface of the cylinder 24.

<Flame Propagation Promotion Operation>

[0065] In one combustion cycle, a flame propagation promotion operation of
increasing a flame propagation speed is performed during a flame
propagation after the ignition operation.

[0066] According to the present embodiment, as the flame propagation
promotion operation, a first operation and a second operation are
performed. In the first and second operations in series, the supply
destination of the microwave is switched in order from the first antenna
41, the second antenna 42, and the third antenna 43 in turn. Here, the
pulse width of the electromagnetic wave drive signal is configured so
that the microwave pulse is continuously outputted until immediately
after the flame surface reaches the wall surface of the cylinder 24.

[0067] During the first operation, the electronic control device 30
outputs a first distribution signal immediately before the flame surface
reaches the emitting end of the second antenna 42. For example, the first
distribution signal is outputted at a timing when the flame surface is
passing through approximately a midpoint between the first and second
antennae 41 and 42. The distributor 33, upon receiving the first
distribution signal, switches the supply destination of the microwave to
the second antenna 42. Then, as shown in FIG. 3b, the microwave is
emitted from the second antenna 42 to the combustion chamber 20, and a
strong electric field region 52 is formed in the vicinity of the emitting
end of the second antenna 42. From the second antenna 42, the microwave
is emitted until immediately after the flame surface has passed through
the strong electric field region 52.

[0068] In the strong electric field region 52, for example, free electrons
discharged from the flame are accelerated. The accelerated free electrons
collide with ambient gas molecules. The gas molecules thus collided are
ionized. Also, free electrons discharged due to the ionization of the gas
molecules are accelerated in the strong electric field region 52, and
ambient molecules are ionized. In this manner, an avalanche-like gas
molecule ionization occurs, and the microwave plasma is generated in the
strong electric field region 52.

[0069] In the strong electric field region 52, active species (such as OH
radical) having strong oxidation power are generated by the microwave
plasma. According to the present embodiment, while the flame is
propagated following the ignition of the fuel air mixture, the active
species are generated in a region not yet reached by the flame surface.
This means that the flame surface passes through the region in which the
active species have been generated. Therefore, oxidation reaction in the
flame surface is promoted by the active species, and the flame
propagation speed is increased. Also, since the microwave plasma in the
strong electric field region 52 is brought into contact with the flame
surface of weakly ionized plasma, the flame surface is supplied with the
energy of the microwave, thereby the flame propagation speed is further
increased.

[0070] Then, in the second operation, the electronic control device 30
outputs a second distribution signal immediately before the flame surface
reaches the emitting end of the third antenna 43. For example, the second
distribution signal is outputted at a timing when the flame surface is
passing through approximately a midpoint between the second and third
antennae 42 and 43. The distributor 33, upon receiving the second
distribution signal, switches the supply destination of the microwave to
the third antenna 43. Then, as shown in FIG. 3c, a strong electric field
region 53 is formed in the vicinity of the emitting end of the third
antenna 43. In the strong electric field region 53, the microwave plasma
is generated. During the second operation, similarly to the first
operation, the microwave plasma is generated in a region not yet reached
by the flame surface, and the flame propagation speed is increased owing
to the microwave plasma.

EFFECT OF EMBODIMENT

[0071] According to the present embodiment, the emitting position of the
first antenna 41, from which the microwave is emitted, is located
downstream of the discharge gap in relation to the direction of the gas
flow 35 at the discharge gap so that the discharge plasma 36 that has
been drifted due to the gas flow 35 is irradiated with the microwave. As
a result of this, the discharge plasma 36 that has been drifted due to
the gas flow 35 is present at a location which the microwave is emitted
to. Therefore, in comparison with a conventional internal combustion
engine that is not configured in view of the fact that the discharge
plasma 36 is drifted, it is possible to increase the energy of the
microwave absorbed by the discharge plasma 36. As a result thereof, it is
possible to reduce the intensity of the reflected microwave.

[0072] Therefore, it is possible to reduce the amount of unburned fuel in
a case in which a lean fuel air mixture is burned.

[0073] Furthermore, according to the present embodiment, it is configured
such that the active species are generated in a region not yet reached by
the flame surface so that the flame surface passes through the region in
which the active species have been generated. Accordingly, oxidation
reaction in the flame surface is promoted by the active species, and it
is possible to improve the propagation speed of the flame surface.

FIRST MODIFIED EXAMPLE OF EMBODIMENT

[0074] According to the first modified example, as shown in FIG. 6, four
antenna groups are provided. The number of antenna groups corresponds to
the number of regions defined between adjacent opening parts 25a and 26a
of the intake and exhaust ports 25 and 26.

[0075] A first antenna group (an antenna group on a right side of the
ignition plug 15 in FIG. 6) is constituted by a first antenna 41, a
second antenna 42, and a third antenna 43. Each of the remaining second
to fourth antenna groups is constituted by a second antenna 42 and a
third antenna 43. The electromagnetic wave emission device 13 is provided
with an electromagnetic wave unit including a power supply for
electromagnetic wave 31, an electromagnetic wave oscillator 32, and a
distributor 33 for each antenna group.

[0076] The first antenna 41 is supplied with a microwave at the same
timing as the first antenna 41 of the embodiment described above. Each
second antenna 42 is spaced apart from the ignition plug 15 at the same
distance as the second antenna 42 of the embodiment described above, and
supplied with a microwave at the same timing as the second antenna 42 of
the embodiment described above. Each third antenna 43 is embedded in the
gasket 18 similarly to the third antenna 43 of the embodiment described
above, and supplied with a microwave at the same timing as the third
antenna 43 of the embodiment described above.

[0077] According to the first modified example, after ignition of the fuel
air mixture by supplying the discharge plasma 36 with the energy of the
microwave emitted from the first antenna 41, the flame propagation speed
is increased owing to the microwave plasma generated by the microwave
emitted from each second antenna 42, and then, another microwave plasma
generated by the microwave emitted from each third antenna 43.

SECOND MODIFIED EXAMPLE OF EMBODIMENT

[0078] According to the second modified example, as shown in FIG. 7, as
antennae for supplying the microwave to regions not yet reached by the
flame surface similarly to the second antennae 42 and the third antennae
43, antennae 27a and 28a are provided on respective valve heads of the
intake and exhaust valves 27 and 28. Transmission lines connecting to
antennae 27a and 28a are provided in respective valve shafts. The
microwave outputted from the electromagnetic wave oscillator 32 is
supplied to the transmission line byway of non-contact power feeding.

THIRD MODIFIED EXAMPLE OF EMBODIMENT

[0079] According to the third modified example, the emitting position of
the antenna for supplying the microwave to the region not yet reached by
the flame surface similarly to the second antenna 42 and the third
antenna 43, is located at a region where occurrence frequency of knocking
is relatively high in the combustion chamber 20. For example, the
emitting position of the antenna may be located outwardly from the
opening part 25a of the intake port 25.

[0080] In the combustion chamber 20, the microwave plasma is generated
before the flame surface reaches to the region where occurrence frequency
of knocking is relatively high, and the active species are generated
along with generation of the microwave plasma. According to the third
modified example, since the active species are generated in a region
where knocking is likely to be caused, it is possible to prevent the
speed of the flame surface from decreasing before the flame surface
reaches the concerned region. Accordingly, since it is possible to cause
the flame to reach the region where knocking is likely to be caused
before knocking occurs, it is possible to suppress occurrence of
knocking.

FOURTH MODIFIED EXAMPLE OF EMBODIMENT

[0081] According to the fourth modified example, as shown in FIG. 8, in
place of the antenna groups of the first modified example, rod shaped
antennae 46 are provided. The antennae 46 extend in respective radial
directions on the ceiling surface of the combustion chamber 20 along the
respective intervening regions each defined by the adjacent opening parts
25a and 26a of the intake and exhaust ports 25 and 26. Each antenna 46
extends slightly outward from the ignition plug 15 straightforwardly
toward the vicinity of the wall surface of the cylinder 24. Here, at
least the antenna 46 disposed between the adjacent opening parts 26a of
the exhaust ports 26 (the antenna on the right side of the ignition plug
15 in FIG. 8) faces toward the flexure part of the discharge plasma 36 at
an inner end thereof.

[0082] The electromagnetic wave emission device 13 is provided with an
electromagnetic wave unit that includes a power supply for
electromagnetic wave 31 and an electromagnetic wave oscillator 32 for
each antenna 46. Each electromagnetic wave unit, unlike the first
modified example, does not include a distributor 33. Instead, each
electromagnetic wave unit includes an electric field adjuster that
changes a location of a strong electric field region, which has an
electric field relatively strong in intensity, on a surface of the
antenna 46. The electric field adjuster is, for example, a stub tuner
that can adjust impedance of a transmission line of the microwave. The
stub tuner is configured to be capable of changing an electrical length
of a stub by, for example, adjusting a location of short-circuiting the
stub to the ground.

[0083] At a time of the ignition operation, each electromagnetic wave unit
causes the electric field adjuster to operate so that the strong electric
field region is located on the inner end surface of the antenna 46. An
emitting position of the antenna 46 disposed between the opening parts
26a of the exhaust ports 26 faces toward the discharge plasma 36 that has
been drifted by the tumble flow 35. Accordingly, the discharge plasma 36
effectively absorbs the energy of the microwave. As a result of this, the
discharge plasma 36 is thickened, and the fuel air mixture is volume
ignited.

[0084] The microwave is continuously emitted from each antenna 46 during
the flame propagation after the fuel air mixture is ignited. The electric
field adjuster moves the emitting position of the microwave on each
antenna 46 outwardly ahead of the flame surface. The region not yet
reached by the flame surface becomes the strong electric field region.
The strong electric field region moves outward, and the microwave plasma
generated by the strong electric field region also moves outward along
with the movement of the strong electric field region. As a result of
this, the flame surface passes through the region in which the active
species are generated, oxidation reaction on the flame surface is
promoted, and thus, the flame propagation speed is improved.

FIFTH MODIFIED EXAMPLE OF EMBODIMENT

[0085] According to the fifth modified example, as shown in FIG. 9, the
distance between the first antenna 41 and the ignition plug 15 is
increased in comparison with the embodiment described above. During the
emission operation at the time of ignition of the fuel air mixture, the
emitting position of the microwave on the first antenna 41 is locates
downstream of the discharge gap in the direction of the gas flow 35 at
the discharge gap. At the time of discharge operation, the microwave
emitted from the first antenna 41 creates a strong electric field region
51 in a region adjacent to the discharge plasma 36 that has been drifted
by the gas flow 35. The second antenna 42 is disposed in a middle
position between the first antenna 41 and an outer periphery of the
ceiling surface of the combustion chamber 20.

[0086] More particularly, in the ignition operation, the first antenna 41
emits the microwave to the combustion chamber 20 during the same period
as the embodiment described above. At the timing of spark discharge, the
strong electric field region 51 is created in the vicinity of the
emitting end of the first antenna 41.

[0087] In a case in which the strong electric field region 51 is not
created, the discharge plasma 36 is extended only to a location shown in
FIG. 9a. On the other hand, in a case in which the strong electric field
region 51 is created, the discharge plasma 36 reacts with an electric
field in the strong electric field region 51, and then, stretches and
enters into the strong electric field region 51, as shown in FIG. 9b. The
discharge plasma 36 effectively absorbs the energy of the microwave, and
forms microwave plasma. According to the fifth modified example, since
the microwave is absorbed by the discharge plasma 36, it is possible to
reduce the intensity of the reflected microwave.

SIXTH MODIFIED EXAMPLE OF EMBODIMENT

[0088] According to the sixth modified example, as shown in FIG. 10, in
addition to the three antennae 41 to 43 of the embodiment described
above, a fourth antenna 44 is provided at the location of the first
antenna of the fifth modified example. During the emission operation, an
emitting position of the microwave on the fourth antenna 44 is located
downstream of the discharge gap, further away from the emitting position
of the first antenna 41 in the direction of the gas flow 35 at the
discharge gap. At the time of the discharge operation, the microwave
emitted from the fourth antenna 44 creates a strong electric field region
in a region adjacent to the discharge plasma 36 that has been drifted by
the gas flow 35.

[0089] More particularly, firstly, the first antenna 41 starts to emit the
microwave in the ignition operation. A spark discharge is caused
immediately after the microwave emission starts. The microwave emission
from the first antenna 41 is maintained until immediately after the spark
discharge. The discharge plasma 36 that has been stretched due to the
tumble flow 35 absorbs the energy of the microwave emitted from the first
antenna 41, and then thickens.

[0090] Subsequently, immediately after the spark discharge, the
distributor 33 switches the supply destination of the microwave to the
fourth antenna 44. In the vicinity of an emitting end of the fourth
antenna 44, the strong electric field region is created. The thickened
discharge plasma 36 reacts with an electric field of the strong electric
field region in the vicinity of the emitting end of the fourth antenna
44. The discharge plasma 36 is supplied with the energy of the microwave
also from the fourth antenna 44. As a result thereof, the fuel air
mixture is volume ignited by the discharge plasma 36. According to the
sixth modified example, it is possible to supply a larger amount of
energy to the discharge plasma 36.

[0091] An emission period of the microwave from the first antenna 41 may
overlap with an emission period of the microwave from the fourth antenna
44. In this case, for example, a first electromagnetic wave oscillator
that supplies a microwave to the first antenna 41, and a second
electromagnetic wave oscillator that supplies a microwave to the fourth
antenna 44 may be provided. Oscillation periods of the microwaves by the
electromagnetic wave oscillators may be the same, or a start and end of
the oscillation period of the second electromagnetic wave oscillator may
be delayed than those of the other.

SEVENTH MODIFIED EXAMPLE OF EMBODIMENT

[0092] According to the seventh modified example, as shown in FIG. 11,
floating electrodes 50, operative to keep the discharge plasma 36
drifting at a constant direction, are provided on an exposed surface of
the cylinder head 22. The exposed surface of the cylinder head 22 is
exposed toward the combustion chamber 20. A pair of the floating
electrodes 50 are provided facing toward each other on opposite sides
with respect to the direction of the gas flow 35 at the discharge gap (a
line connecting the ignition plug 15 and the first antenna 41). Each
floating electrode 50 is insulated from the cylinder head 22 by an
insulating member 60.

EIGHTH MODIFIED EXAMPLE OF EMBODIMENT

[0093] According to the eighth modified example, as shown in FIG. 12, on
the ceiling surface of the combustion chamber 20, a plug for microwave 70
is provided at a center of the combustion chamber 20 and a discharge
electrode 71 is provided on a left side of the plug for microwave 70 in
FIG. 12.

[0094] The plug for microwave 70 constitutes a coaxial line, and is
provided with a central conductor 70a, an outer conductor 70b, and an
insulator 70c. The plug for microwave 70 is supplied with a microwave
pulse oscillated by the electromagnetic wave oscillator 32. A ground
electrode 73 that forms a discharge gap along with the discharge
electrode 71, which will be described later, is connected to the outer
conductor 70b of the plug for microwave 70. The ground electrode 73 is
formed in a shape of a plate and protruded from an edge surface of the
outer conductor 70b. As the plug for microwave 70, a type of an ignition
plug having a ground electrode sharply bent (type of an ignition plug
having a ground electrode protruded from the outer conductor in an axial
direction of the ignition plug and sharply bent in a manner as facing
toward a central electrode), from which a tip end part of the ground
electrode is omitted, may be employed.

[0095] The discharge electrode 71 is formed in a shape of a plate or a
rod. The discharge electrode 71 is insulated from the cylinder head 22 by
an insulator 72. The discharge electrode 71 and the ground electrode 73
are protruded from the ceiling surface of the combustion chamber 20. In
comparison between the discharge electrode 71 and the ground electrode
73, protrusion length of the discharge electrode 71 is longer than that
of the ground electrode 73. The discharge electrode 71 is supplied with a
high voltage pulse outputted from the ignition coil 14. Then, discharge
plasma 36 is generated at the discharge gap. Here, the discharge
electrode 71 may be bent sharply toward a side of the ground electrode 73
so that a tip end of the discharge electrode 71 is located closest to the
ground electrode 73.

[0096] According to the eighth modified example, the discharge gap is
formed upstream of the plug for microwave 70 in the direction of the gas
flow 35 in the vicinity of a tip end of the plug for microwave 70. The
discharge plasma 36 generated at the discharge gap is drifted toward the
plug for microwave 70 due to the gas flow 35. A tip end surface of the
plug for microwave 70 faces toward the discharge plasma 36 that has been
drifted due to the gas flow 35. The discharge plasma 36 that has been
drifted due to the gas flow 35 is present at a location which the
microwave is emitted to. Accordingly, it is possible to cause the
discharge plasma 36 to effectively absorb the energy of the microwave.

[0097] Furthermore, according to the eighth modified example, since an
ignition location (an ignition region) of the fuel air mixture is close
to a central part of the combustion chamber 20, it is possible to improve
uniformity in diffused state of the flame, thereby making it possible to
reduce an amount of unburned fuel air mixture. Furthermore, since the
plug for microwave 70 is disposed at a central part of the ceiling
surface of the combustion chamber 20, it is possible to increase the
cross section area of the plug for microwave 70, thereby making it
possible to reduce reflection of the microwave at the tip end part of the
plug for microwave 70.

OTHER EMBODIMENTS

[0098] The embodiment described above may also be configured as follows.

[0099] According to the embodiment described above, the microwave may be
emitted to a region which the flame surface has been passed through so as
to generate microwave plasma, thereby promoting the propagation of the
flame surface from behind the flame surface. For example, even after the
flame surface has passed through between the second antenna 42 and the
third antenna 43, the microwave emission from the second antenna 42 may
be continued so as to maintain the microwave plasma in the vicinity of
the emitting end of the second antenna 42. In this case, temperature
rises owing to the microwave in the region which the flame has passed
through, and accordingly, oxidation reaction of the fuel air mixture is
promoted. Furthermore, since pressure rises behind the flame surface,
flame propagation is promoted.

[0100] Furthermore, in the embodiment described above, the emitting
position of the microwave on each of antennae 41 to 43 may be covered by
an insulator or dielectric material.

INDUSTRIAL APPLICABILITY

[0101] The present invention is useful in relation to an internal
combustion engine provided with an electromagnetic wave emission device
that emits an electromagnetic wave to a combustion chamber.